Since the advent of scanning tunneling microscopy and atomic force microscopy, intensive research interest in the field of nanometer and submicrometer length scale surface modifications utilizing the above-mentioned techniques has been expressed. These fabrications usually involve controlled modifications of the surface morphologies which are critical and important for the fabrication of miniaturized micromechanical and electronic devices with nanometer scale dimensions. It becomes more significant when one considers the limitations encountered by the photolithography systems as device dimensions are constantly being scaled down to the submicron regime. The lack of efficient light source past the ArF laser line at 193 nm and the unavailability of good optical materials may limit prompt application of such photolithography systems. Although other techniques like the e-beam and X-ray lithography may serve as attractive alternatives, precise optimization of process parameters and technical-related issues may further hinder their participation.
Attempts to achieve small features using nanolithography systems have been described. For example, U.S. Pat. Nos. 5,517,280 and 5,666,190, disclose the use of a scribing tool (in this case a photolithographic wafer) with an array of cantilevers and tips to create features in a fabrication wafer. Waveguides extend longitudinally along the bottom surface of the cantilevers, and an aperture in the waveguide is placed at the apex of a tip on the end of each cantilever. The photolithographic wafer is brought into close proximity with the fabrication wafer such that a gap separates the tip from the surface of the fabrication wafer. A piezoresistor coupled to each cantilever allows the resonance frequency of the cantilever to vary according to the size of the gap. In order to obtain a uniformly suitable gap between every tip and the fabrication wafer i.e. to get a uniform tip height, a capacitive plate is provided above the cantilever. A DC electric field between the capacitive plate and the fabrication wafer causes the thicker portion of the cantilever to flex. The cantilever continues to flex until the gap is adjusted to the proper level, indicating the correct spacing for proper exposure of photoresist on the surface of wafer. Although this photolithography method allows a plurality of cantilevers to operate within the same photolithographic wafer, there are the following limitations to the method. Firstly, the fabrication of the cantilevers and waveguides is a complex process and therefore expensive. In addition, due to the complex fabrication process, the resulting cantilevers and tips within the same wafer are often not uniform, resulting in difficulties in obtaining uniform alignment of tip heights. This problem is partially solved by providing the capacitive plates to allow for individual adjustment of each cantilever. However, this means that numerous electrical connections and capacitive plates have to be provided along the surface of the photolithographic wafer. This not only makes the circuitry extremely complex, but also limits the number of cantilevers which can be realistically controlled within one photolithographic wafer. Secondly, if a cantilever has to be flexed to a relatively large degree before the desired gap distance can be reached, the tip may be positioned to face the fabrication wafer at an angle substantially different from normal. This may further affect the lithographic step when the reactive light is applied to photoresist surface.
U.S. Pat. No. 4,991,962 discloses a high precision alignment system for aligning a mask with a wafer in a high resolution nanolithography system using optical methods. The alignment is accomplished by observing alignment targets such as multiple diffraction gratings on the mask and the wafer to generate interference signals. It is clear that although alignment obtained using this method is in the order of a few nanometers, this method only allows two dimensional alignment. When a third dimension of alignment is required, such as for multiple tip scribing wafers, this method would not be useful for preventing tilting of scribing tools.